Sources of fuel useful for heating, transportation, and the production of petrochemicals are becoming increasingly scarce and therefore more costly. Industries such as those producing energy and petrochemicals are actively searching for cost effective fuel feed stock alternatives for use in generating those products and many others. Additionally, due to the ever increasing costs of fossil fuels, transportation costs for moving fuel feed stocks for production of energy and petrochemicals is rapidly escalating.
These energy and petrochemical producing industries, and others, normally have relied on the use of fossil fuels, such as coal, oil and/or natural gas, for use in combustion and gasification processes for the production of energy for heating and electricity, and the generation of synthesis gas used for the downstream production of petrochemicals and liquid fuels.
Combustion and gasification are thermochemical processes that are used to release the energy stored within the fuel source. Combustion takes place in a reactor in the presence of excess air, or an excess of oxygen. Combustion is generally used for generating steam which in turn is used to generate electricity through steam turbines. However, the brute force nature of complete combustion of fuel causes significant amounts of pollutants to be generated in the gas given off during combustion. For example, combustion in an oxidizing atmosphere of, for example, coal releases nitrogen oxides, a precursor to ground level ozone which can stimulate asthma attacks. Combustion of high sulfur containing fossil fuels, such as coal, is also the largest source of sulfur dioxide which in turn produces sulfates that are very fine particulates. Fine particle pollution from U.S. power plants cuts short the lives of over 30,000 people each year. Hundreds of thousands of Americans suffer from asthma attacks, cardiac problems and upper and lower respiratory problems associated with fine particles from power plants.
Gasification also takes place in a reactor, although in the absence of air, or in the presence of substochiometric amounts of oxygen. The thermochemical reactions that take place in the absence of oxygen or under substochiometric amounts of oxygen do not result in the formation of nitrogen oxides or sulfur oxides.
Gasification generates a gaseous fuel rich product. During gasification, two processes take place that convert the fuel source into a useable fuel gas. In the first stage, pyrolysis or flaming pyrolysis, the feed stock releases volatile components of the fuel at temperatures below 600° C. (1112° F.); a process known as devolatization. The by-product of pyrolysis that is not vaporized is called char and consists mainly of fixed carbon and ash. In the second gasification stage, the carbon remaining after pyrolysis undergoes a reduction processes by reacting either with steam and/or hydrogen. Gasification with pure oxygen results in a high quality mixture of carbon monoxide and hydrogen and virtually no nitrogen.
The basic gasification reactions that occur are:                1) C+1/2O2→CO −110.5 kJ/mol (exothermic)        2) C+H2O→CO+H2 +131 kJ /mol (endothermic)        3) C+CO2→2CO +172 kJ/mol (endothermic)        4) C+2H2→CH4 −74 kJ/mol (exothermic)        5) CO+H2O →CO2+H2 −41 kJ/mol (exothermic)        6) CO+3H2→CH4+H2O −205 kJ/mol (exothermic)        
All of these reactions are reversible and their rates depend on the reaction kinetics, which are functions of temperature, pressure and concentration reactants in the reactor. Heat can be supplied directly or indirectly to satisfy the requirements of the endothermic reactions.
A variety of gasifier types have been developed. They can be grouped into four major classifications: fixed-bed updraft, fixed-bed downdraft, bubbling fluidized-bed and circulating fluidized bed. Differentiation is based on the means of supporting the fuel source in the reactor vessel, the direction of flow of both the fuel and oxidant, and the way heat is supplied to the reactor.
The updraft gasifier, also known as counterflow gasification, is the oldest and simplest form of gasifier; it is still used for coal gasification. The fuel is introduced at the top of the reactor, and a grate at the bottom of the reactor supports the reacting bed. The oxidant in the form of air or oxygen and/or steam are introduced below the grate and flow up through the bed of fuel and char. In an ideal gasifier, complete conversion of char would occur at the bottom of the bed, liberating CO2 and H2O. These hot gases (˜1000° C.) pass through the bed above, where they are reduced to H2 and CO and cooled to 750° C. Continuing up the reactor, the reducing gases (H2 and CO) pyrolyse the descending dry fuel and finally dry any incoming wet fuel, leaving the reactor at a low temperature (˜500° C.). The advantages of updraft gasification are: simple, higher thermal efficiency, low cost process that is able to handle fuel with a high moisture and high inorganic content. The primary disadvantage of updraft gasification is that the synthesis gas contains 10-20% tar by weight, requiring extensive syngas cleanup before engine, turbine or synthesis applications.
The downdraft gasification, also known as cocurrent-flow gasification, has the same mechanical configuration as the updraft gasifier except that the oxidant and product gases flow down the reactor, in the same direction as the fuel. A major difference is that this process can crack up to 99.9% of the tars formed. Low moisture fuel (<20%) and air or oxygen are ignited in the reaction zone at the top of the reactor. The flame generates pyrolysis gas/vapor, which burns intensely leaving 5 to 15% char and hot gas. These gases flow downward and react with the char at 800 to 1200° C., generating more CO and H2 while being cooled to below 800° C. Finally, unconverted char and ash pass through the bottom of the grate and are sent to disposal. The advantages of downdraft gasification are: up to 99.9% of the tar formed is consumed, requiring minimal or no tar cleanup Minerals remain with the char/ash, reducing the need for a cyclone. The disadvantages of downdraft gasification are: requires feed drying to a low moisture content (<20%). The syngas exiting the reactor is at high temperature, requiring a secondary heat recovery system; and 4-7% of the carbon remains unconverted.
The bubbling fluidized bed consists of fine, inert particles of sand or alumina, which have been selected for size, density, and thermal characteristics. As gas (oxygen, air or steam) is forced through the bed of particles, a point is reached when the frictional force between the particles and the gas counterbalances the weight of the solids. At this gas velocity (called minimum fluidization velocity), bubbling and channeling of gas through the media may occur, such that the particles remain in the reactor and appear to be in a “boiling state”. The fluidized particles tend to break up the biomass fed to the bed and ensure good heat transfer throughout the reactor. The advantages of bubbling fluidized-bed gasification are: yields a uniform product gas; exhibits a nearly uniform temperature distribution throughout the reactor; able to accept a wide range of fuel particle sizes, including fines; provides high rates of heat transfer between inert material, fuel and gas; high conversion possible with low tar and unconverted carbon. The disadvantages of bubbling fluidized-bed gasification are: lower gas-solid contact efficiency due to formation of bubbles, and increased attrition and dust entrainment rates. large bubble size may result in gas bypass through the bed.
The circulating fluidized bed gasifiers operate at gas velocities higher than the so-called transport velocity, resulting in significant entrainment of the particles in the gas stream. Thus the entrained particles in the gas exited from the top of the reactor must be-separated in a cyclone and returned to the reactor. The advantages of circulating fluidized-bed gasification are: it is suitable for rapid reactions; high heat transport rates possible due to high heat capacity of bed material; high conversion rates possible with low tar and unconverted carbon. It also makes production of higher energy content syngas possible because heat required for gasification can be transferred from outside through circulating particles acting as heat carriers. The disadvantages of circulating fluidized-bed gasification are: temperature gradients occur in the direction of solid flow; smaller particles are required; high velocities may result in equipment erosion; and heat exchange is less efficient than bubbling fluidized-bed.
Normally these gasifiers use a homogeneous source of fuel because a constant unchanging fuel source allows the gasifier to be designed optimally for this particular fuel, for production of a desired product. Common types of fuel used today in gasifiers are wood, coal, petroleum, and, biomass. Since some of these fuel sources are becoming increasingly more expensive, energy and petrochemical suppliers are seeking alternative fuel feed stocks.
One potential source of a large amount of feed stock for gasification is waste. Waste such as municipal solid waste is presently often disposed of or used in incineration processes to generate heat and/or steam for use in -turbines. Incineration is a combustion process and the negative drawbacks for combustion have been described above.
One of the most significant threats facing the environment today is the release of greenhouse gases (GHGs) into the atmosphere. GHGs such as carbon dioxide, methane, nitrous oxide, water vapor, carbon monoxide, nitrogen oxide, nitrogen dioxide, and ozone, absorb heat from incoming solar radiation but do not allow long-wave radiation to reflect back into space. GHGs in the atmosphere result in the trapping of absorbed heat and warming of the earth's surface. In the U.S., GHG emissions come mostly from energy use driven largely by economic growth, fuel used for electricity generation, and weather patterns affecting heating and cooling needs. Energy-related carbon dioxide emissions, resulting from petroleum and natural gas, represent 82 percent of total U.S. human-made GHG emissions. Another greenhouse gas, methane, comes from landfills, coal mines, oil and gas operations, and agriculture; it represents nine percent of total emissions. Nitrous oxide (5 percent of total emissions), meanwhile, is emitted from burning fossil fuels and through the use of certain fertilizers and industrial processes. World carbon dioxide emissions are expected to increase by 1.9 percent annually between 2001 and 2025. Much of the increase in these emissions is expected to occur in the developing world where emerging economies, such as China and India, fuel economic development with fossil energy. Developing countries' emissions are expected to grow above the world average at 2.7 percent annually between 2001 and 2025; and surpass emissions of industrialized countries near 2018.
Landfills can also be significant sources of GHG emissions if no or poor landfill gas connection system is in place, mostly because of methane released during decomposition of waste, such as, for example, municipal solid waste (MSW). Compared with carbon dioxide, methane is twenty-one times stronger than carbon dioxide as a GHG. Today, landfills are responsible for about 4% of the anthropogenic emissions. Considerable reductions in methane emissions can be achieved by combustion of waste and by collecting methane from landfills. The methane collected from the landfill can either be used directly in energy production or flared off, i.e., eliminated through combustion without energy production. Combustion Of Waste May Reduce Greenhouse Gas Emissions, Science Daily (Dec. 8, 2007).
One measure of the impact human activities have on the environment in terms of the amount of green house gases produced is the carbon footprint, measured in units of carbon dioxide (CO2). The carbon footprint can be seen as the total amount of carbon dioxide and other GHGs emitted over the full life cycle of a product or service. Normally, a carbon footprint is usually expressed as a CO2 equivalent (usually in kilograms or tons), which accounts for the same global warming effects of different GHGs. Carbon footprints can be calculated using a Life Cycle Assessment method, or can be restricted to the immediately attributable emissions from energy use of fossil fuels.
An alternative definition of carbon footprint is the total amount of CO2 attributable to the actions of an individual (mainly through their energy use) over a period of one year. This definition underlies the personal carbon calculators. The term owes its origins to the idea that a footprint is what has been left behind as a result of the individual's activities. Carbon footprints can either consider only direct emissions (typically from energy used in the home and in transport, including travel by cars, airplanes, rail and other public transport), or can also include indirect emissions which include CO2 emissions as a result of goods and services consumed, along with the concomitant waste produced.
The carbon footprint can be efficiently and effectively reduced by applying the following steps: (i) life cycle assessment to accurately determine the current carbon footprint; (ii) identification of hot-spots in terms of energy consumption and associated CO2-emissions; (iii) optimization of energy efficiency and, thus, reduction of CO2-emissions and reduction of other GHG emissions contributed from production processes; and (iv) identification of solutions to neutralize the CO2 emissions that cannot be eliminated by energy saving measures. The last step includes carbon offsetting, and investment in projects that aim at the reducing CO2 emissions.
The purchase of carbon offsets is another way to reduce a carbon footprint. One carbon offset represents the reduction of one ton of CO2-eq. Companies that sell carbon offsets invest in projects such as renewable energy research, agricultural and landfill gas capture, and tree-planting.
Purchase and withdrawal of emissions trading credits also occurs, which creates a connection between the voluntary and regulated carbon markets. Emissions trading schemes provide a financial incentive for organizations and corporations to reduce their carbon footprint. Such schemes exist under cap-and-trade systems, where the total carbon emissions for a particular country, region, or sector are capped at a certain value, and organizations are issued permits to emit a fraction of the total emissions. Organizations that emit less carbon than their emission target can then sell their “excess” carbon emissions.
For many wastes, the disposed materials represent what is left over after a long series of steps including: (i) extraction and processing of raw materials; (ii) manufacture of products; (iii) transportation of materials and products to markets; (iv) use by consumers; and (v) waste management. At virtually every step along this “life cycle,” the potential exists for GHG impacts. Waste management affects GHGs by affecting energy consumption (specifically, combustion of fossil fuels) associated with making, transporting, using, and disposing the product or material that becomes a waste and emissions from the waste in landfills where the waste is disposed.
Traditionally, attempts have been made to use various types of incineration as means for reducing the amount, or volume, of materials which must be disposed of in landfills. However, only few have provided economically affordable improvements or effective solutions to the solid waste problems. Of course, incineration, although reducing the volume of wastes disposed of in landfills, creates a large GHG emission and thereby the carbon foot print of the disposed of product, now waste, is not decreased. Various attempts have been made to utilize solid wastes directly, blended with other solid/liquid fuels, or after some form of processing, as fuels for electric power generation. While some projects have proven technically feasible, only a few have proven to be either environmentally desirable or economically attractive. Most waste energy recovery projects cost the municipalities more than the original landfills they replace, and do not represent substantial environmental improvements.
Incineration typically reduces the volume of the MSW by about 90% with the remaining 10% of the volume of the original MSW often being landfilled. This incineration process produces large quantities of the GHG CO2. Typically, the joules of energy produced per equivalents CO2 expelled during incineration are very low. Thus, incineration of MSW for energy production releases GHG into the atmosphere with comparatively little energy return. Therefore, if GHGs are to be avoided, new solutions for the disposal of wastes, such as MSW, other than landfilling and incineration, are needed.
Each material disposed of as waste has a different GHG impact depending on how it is made and disposed. The most important GHGs for waste management options are carbon dioxide, methane, nitrous oxide, and perfluorocarbons. Of these, carbon dioxide (CO2) is by far the most common GHG emitted in the US. Most carbon dioxide emissions result from energy use, particularly fossil fuel combustion. Carbon dioxide is the reference gas for measurement of the heat-trapping potential (also known as global warming potential or GWP). By definition, the GWP of one kilogram (kg) of carbon dioxide is 1. Methane has a GWP of 21, meaning that one kg of methane has the same heat-trapping potential as 21 kg of CO2. Nitrous oxide has a GWP of 310. Perfluorocarbons are the most potent GHGs with GWPs of 6,500 for CF4 and 9,200 for C2F6. Emissions of carbon dioxide, methane, nitrous oxide, and perfluorocarbons are usually expressed in “carbon equivalents.” Because CO2 is 12/44 carbon by weight, one metric ton of CO2 is equal to 12/44 or 0.27 metric tons of carbon equivalent (MTCE). The MTCE value for one metric ton of each of the other gases is determined by multiplying its GWP by a factor of 12/44 (The Intergovernmental Panel on Climate Change (IPCC), Climate Change 1995: The Science of Climate Change, 1996, p. 121). Methane (CH4), a more potent GHG, is produced when organic waste decomposes in an oxygen free (anaerobic) environment, such as a landfill. Methane from landfills is the largest source of methane in the US.
Treatment methods for biodegradable waste—composting and digestion reduce the GHG emissions compared with landfilling. Biogas production in a digestion plant yields more emission reductions than composting, if the biogas can be utilized for production of heat, electricity, or transportation fuel. The efficiency is even better if the separation of waste components takes place already at the source and if fossil fuels are replaced by biogas.
The greater GHG emission reductions are usually obtained when recycled waste materials are processed and used to replace fossil fuels. If the replaced material is biotic (material derived from living organisms), it is not always possible to obtain reductions of emissions. Even other factors, such as the treatment of the waste material and the fate of the products after the use, affect the emission balance. For example, the recycling of oil-absorbing sheets made of recycled textiles lead to emission reductions compared with the use of virgin plastic. In another example, the use of recycled plastic as raw material for construction material was found to be better than the use of impregnated wood. This is because the combustion of plastic causes more emissions than impregnated wood for reducing emissions. If the replaced material had been fossil fuel-based, or concrete, or steel, the result would probably have been more favorable to the recycling of plastic.
Given the effect of GHGs on the environment, different levels of government are considering, and in some instances have initiated, programs aimed at reducing the GHGs released into the atmosphere during the conversion of fuels into energy. One such initiative is the Regional Greenhouse Gas Initiative (RGGI). RGGI is a market-based program designed to reduce global warming pollution from electric power plants in the Northeast. Other such initiatives are being considered in different sections of the U.S. and on the federal level. RGGI is a government mandated GHG trading system in the Northeastern U.S. This program will require, for example, that coal-fired power plants aggressively reduce their GHG emissions by on average 2.5% per year. One way to do this is by changing the fuel source used or scrubbing the emissions to remove the pollutants. An alternative is to purchase carbon credits generated by others which can offset their emissions into the atmosphere.
Thus, there is a need for alternative fuels that burn efficiently and cleanly and that can be used for the production of energy and/or raw materials for the production of petrochemicals. There is, at the same time, a need for waste management systems that implement methods for reducing GHG emissions of waste. In particular, there is a need for reducing the carbon foot print of materials by affecting their end-stage life cycle management. By harnessing and using the energy content contained in waste, it is possible to reduce GHG emissions generated during the processing of wastes and effectively use the waste generated by commercial and residential consumers.
It is therefore an object of this invention to provide an improved and economical process for the disposal of domestic waste by recovering the energy and matter bound within it and reducing the need for fossil fuels. It is a further object of this invention to provide an improved feed stock for the control of the output from processes for the production of energy and/or production of raw materials for petrochemical production. It is another object of this invention to provide an improved feed stock for thermal-conversion of carbon-containing materials to obviate the disadvantages of prior art systems.
It is also an object of this invention to provide an integrated bunker storage system for waste streams based on physical and/or chemical properties of waste that, when subject to chemical conversion, effect the output from processes for the production of energy and/or production of raw materials for petrochemical production. It is another object of this invention to provide an integrated bunker storage system for waste streams based on the chemical content of the waste and based on the use of the waste as components for a blended feed stock in a chemical conversion process. It is yet a further object of this invention to provide an integrated bunker storage system for waste streams based on the energy content of waste streams. It is a further object of this invention to provide a process for storing waste streams based on the desired energy content in feed streams necessary to achieve optimum thermal-conversion.